Streptococcus pneumoniae is a Gram-positive, encapsulated bacterium that is a main cause of infections of the respiratory tract and can lead to severe invasive pneumococcal disease (IPD). More than 90 different pneumococcal serotypes have been described to date. These are classified by the structure of their capsular polysaccharide (CPS), which is unique to each serotype. Consequently, the immune response generated against the CPS varies between different serotypes. This is used to generate specific antibodies in rabbits against the antigen of each serotype. Cross-reactivity between these specific antibodies and other serotypes than those they were raised against is often observed, due to structural similarities of the CPS of different serotypes. Due to its immunological properties, CPS is used as the main component of S. pneumoniae vaccines.
The first efficient vaccine that contained the CPS of four different serotypes was described in 1945. It then took over thirty years until a vaccine was introduced that covered 14 serotypes, shortly followed by a 23-valent vaccine. However, these polysaccharide vaccines had several shortcomings. They were not able to elicit a long-lasting protection and were not effective in the populations most vulnerable to infection, namely children under two years of age as well as immunodeficient and elderly patients. These shortcomings result from the immunology of carbohydrates and were overcome by the introduction of carbohydrate-protein conjugate vaccines. The first pneumococcal conjugate vaccines were the seven-valent (PCV-7) and 10-valent (PCV-10) vaccine. PCV-7 was later replaced with the most recent vaccine (PCV-13), which contains the CPS-glycoconjugates of 13 different serotypes.
The currently marketed vaccines are effective in North America and Europe for individuals of a particular age. The manufacturing process for these vaccines is complex and results in a higher price. Therefore, the vaccine is unaffordable in most developing countries. It is the object of the present invention to provide affordable synthetic saccharide vaccines that contain one of the most prevalent serotypes of the developing world.
Streptococcus pneumoniae serotype 8 has been associated with serotype replacement and cases of invasive pneumococcal disease increased steadily in the last decades.
Streptococcus pneumoniae type 8 is one of the prevalent S. pneumoniae serotypes. The structure of the native Sp8 capsular polysaccharide repeating unit is a tetrasaccharide with the sequence →4)-Glcα-(1→4)-Galα-(1→4)-GlcAβ-(1→4)-Glcβ(1→(J. Am. Chem. Soc. 1957, 79 (11), 2787):

Interestingly, the native serotype 8 tetrasaccharide repeating unit harbors the disaccharide repeating unit of serotype 3. Consequently, cross-reactivity has been found between sera against serotypes 3 and 8, although precipitation of antibodies by the respective heterologous polysaccharide is incomplete. Insufficient epitope overlap may be the reason why capsular polysaccharides (CPS) from both serotypes were included in the 23-valent polysaccharide vaccine despite extensive considerations about cross-reactivity before manufacturing this vaccine.
WO 9640225 A1 provides a Streptococcus pneumoniae serotype 8 oligosaccharide protein conjugate consisting of mixtures of 2 to 4 repeating units coupled using EDC to tetanus toxoid. The mixtures of 2 to 4 repeating units are obtained by non-selective cleavage of the capsular polysaccharide followed by size exclusion chromatography. It is well known that oligosaccharides obtained by degradation of capsular polysaccharides present a high degree of heterogeneity due to co-isolated impurities and new epitopes that are introduced during degradation or as part of an immune evasion strategy by the pathogen. In the case of WO 9640225 A1, this heterogeneity is increased by the non-selective cleavage (TFA, 100° C.), which leads to a mixture of at least 4 saccharides for a fraction containing two repeating units. Thus, oligosaccharides isolated from capsular polysaccharides comprise a set of potentially immunogenic epitopes depending on structural determinants that are recognized as “non-self” by the host immune system. Therefore, a drawback of the immunization with vaccines based on conjugates of isolated oligosaccharides, such as the conjugates of WO 9640225 A1, is the generation of a polyclonal immune response that is directed towards multiple immunogenic epitopes, among which some are non-protective (i.e. the antibodies that are elicited do not protect from diseases associated with S. pneumoniae type 8) or are immunodominant. Such polyclonal immune response leads to toxicities that are not acceptable for a vaccine that is prophylactically administered to healthy (often infant) populations. This drawback is overcome by the pure saccharide of the present invention, which presents a well-defined structure, a degree of purity suitable for clinical applications and no batch-to-batch variability.
Moreover, WO 9640225 A1 discloses that a mixture of oligosaccharides containing 1 repeating unit i.e. a mixture of tetrasaccharides do not contain an immunogenic epitope and that at least mixtures of octasaccharides are required for preparing a conjugate against S. pneumoniae type 8. Surprisingly, the inventors have found that even short saccharides described herein contain a protective glycan epitope and are able to induce a protective immune response against S. pneumoniae serotype 8 bacteria in a human and/or animal host.
Thus, it is the objective of the present invention to provide a pure synthetic saccharide of general formula (I) that is related to the capsular polysaccharide of Streptococcus pneumoniae serotype 8 and contains a protective immunogenic glycan epitope i.e. a glycan epitope that is elicits and is recognized by antibodies which protect against diseases caused by S. pneumoniae type 8. Said saccharide is suitable to be conjugated to an immunogenic carrier to provide a conjugate and a pharmaceutical composition thereof that are useful for prevention and/or treatment of diseases associated with Streptococcus pneumoniae, and more specifically against diseases associated with Streptococcus pneumoniae serotype 8. Furthermore, the synthetic saccharide of general formula (I) is useful as marker in immunological assays for detection of antibodies against Streptococcus pneumoniae bacteria.
The objective of the present invention is solved by the teaching of the independent claims. Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description, the figures, and the examples of the present application.